The transmission and reception subsystems produced for example using MMIC (Monolithic Microwave Integrated Circuits) technology, generally comprise one or more gain control cells that are used to adapt, for example in the case of a radiofrequency transmission subsystem, the desired output level and, in the case of a reception subsystem, the gain of the subsystem at the received signal level.
In practice, a gain control of the transmission and/or reception subsystem is sought that offers a minimum of distortion of the signal processed by the subsystem and of degradation of the noise figure.
Currently, there are a number of solutions in the state of the art of integrated circuits for adjusting the level of the signal in a microwave subsystem:                either by the use of an amplifier that will be switched depending on whether or not amplification of the signal is desired in the subsystem,        or by the use of an attenuator that will be activated or not activated depending on whether it is necessary or not necessary to weaken the signal in the microwave subsystem.        
The choice of amplification or attenuation of the signal in the subsystem generally leads to the amplifier or attenuator being inserted between two simultaneously-controlled switches making it possible to choose one channel out of two for the signal so as to activate or not activate the function concerned.
FIGS. 1a and 1b show gain adjusting electronic devices of the state of the art in a microwave subsystem. These gain adjusting devices are also respectively designated by the terms “gain terminals” and “attenuation terminals”.
FIG. 1a shows the case of use of a microwave amplifier G 10 between two switches simultaneously controlled in order to make it possible to choose one channel out of two for the signal, an input switch C1 and an output switch C2. The common points 12 of the switches C1, C2 are respectively linked, one to the input E of the device and the other to the output S of the device.
The switches C1 and C2, set to the amplification position, respectively apply an input signal Ue received at the input E of the electronic gain adjusting device, to an input eA of the amplifier and an output signal Us supplied by an output sA of the amplifier to the output S of the device.
In position of transmission without amplification of the input signal Ue to the output S of the device, the switches C1 and C2 directly link the input E to the output S via a conductor 20, therefore without any amplification but with transmission losses imparted by the switches and the electrical connections.
FIG. 1b shows a gain adjusting device with the switches C1, C2 as represented in FIG. 1a but using an attenuator Att 24 in place of the amplifier G 10.
In the case where these gain adjusting devices of the state of the art are placed close to a receiving antenna, for example to drive a radiofrequency receiver, the amplifier device of FIG. 1a is given preference so as not to degrade the noise figure NF of the receiver.
Nevertheless, the use of switches C1, C2 to produce the function of FIG. 1a leads, through the high-frequency losses that they introduce, to a degradation of the noise figure NF of the receiver. In practice, since the switches are passive structures, the losses of the input switch C1 (expressed in dB) are directly added to the noise figure NF (in dB) of the rest of the device, in other words the amplifier G 10 and the switch C2 linked to the output S of the device.
Furthermore, the use of two switches C1 and C2 to produce these gain adjusting devices leads to an increase in the losses of the microwave subsystem regardless of whether the device (or the path travelled by the signal in the subsystem) is configured to maximum gain or to minimum gain. In practice, of the two devices providing gain adjusting functions in FIGS. 1a and 1b, only the losses of the switches C1 and C2 in the attenuation mode of FIG. 1b can be compensated by an equivalent reduction in the value of the attenuator Att, but this is then done to the detriment of the gain difference between the two switched channels (direct transmission channel via the conductor 20 and attenuation channel via the attenuator Att).
FIG. 1c shows a gain adjusting device with switches C1, C2 as represented in FIGS. 1a and 1b also comprising two switched channels. The first channel contains an amplifier G 10, the second an attenuator Att 24. This solution makes it possible to increase, if necessary, the gain difference between the two states of the device but changes nothing with the drawback of increasing the noise figure due to the losses of the input switch C1.
In the case of a radiofrequency reception subsystem that uses a number of gain adjusting devices to manage the dynamics of the signal in the radiofrequency subsystem, it is ultimately necessary to add one or more other amplifiers to compensate all the losses of these devices.
In the state of the art of radiofrequency receivers, there is no ideal solution for avoiding a degradation of the noise figure.
However, to obtain a good noise figure NF, the reception subsystems of the state of the art usually include a low-noise amplifier (LNA) at the head of the reception subsystem, without the possibility of switching it, which has the drawback, in the case of the reception of strong signals, of introducing distortions of the received signal but also a degradation of the characteristics of the receiver, even destruction of said receiver.